The invention relates to an apparatus and method for accurately computing an area on a soft three-dimensional object from two-dimensional image data obtained by means of a camera, such as one associated with a digital colposcope; and more specifically, this invention relates to the accurate measurement of lesions of a cervix from two-dimensional images of the cervix.
The HPV virus (human papilloma virus), which produces genital warts, is well known to cause or accelerate development of pre-cancerous or cancerous tissue of the cervix. HPV is a sexually transmitted disease, the widespread incidence of which is approaching epidemic proportions.
Early detection of HPV lesions allows simple, effective treatment by laser surgery, freezing, or chemical exfoliation. The initial appearance of an HPV lesion ordinarily does not call for any surgical treatment, because small lesions frequently disappear if left alone because of the action of a woman's immune system, if it is healthy. Most gynecologists make decisions whether to surgically treat or otherwise remove the lesions based on whether they believe that the lesion is growing or shrinking. However, it is very difficult to accurately determine if the lesion is growing or shrinking.
Previous techniques for determining the progression of a lesion of the cervix include forming of a subjective opinion by the examining physician as to whether the lesion size has changed since a previous examination based on the physician's viewing of the lesion. Typically, present photographs of the lesion, made by means of a colposcope, are compared with a corresponding prior photograph. More recently, computerized measurement of the area has been performed using a colposcope to obtain a video image of the cervix and processing the video image data. However, measurements or computations of the area of a three-dimensional object from a two-dimensional image such as a photograph or a digitized two-dimensional image taken by means of a CCD camera inherently contain inaccurate representations of lesion areas if the lesions are growing on sloped areas of the cervix, in which case a particular lesion may be interpreted as unchanging or decreasing in size when actually it is increasing in size and hence may be dangerous.
Colposcopy is a well known technique of using a colposcope for visualizing the cervix under magnification. By using a speculum to spread the walls of the vagina to allow direct illumination and visualization of the cervix, a gynecologist can use a colposcope to directly visualize HPV lesions. However, what the gynecologist sees using the colposcope is a two-dimensional view of a three-dimensional lesion. Depending on the viewing angle of the colposcope relative to the slope of the portion of the cervix on which the lesion occurs, the physician often can see only a "side view" of the lesion which usually prevents an accurate estimate or measurement of how large the surface area of the lesion is. In this case, large increases in the area of the HPV lesion over a period of time cannot be reliably determined from the two-dimensional views of the lesion permitted by the colposcope at the beginning and end of such period of time.
The closest prior art colposcopes, called "digital imaging colposcopes", were developed by the present inventors. A digital imaging colposcope projects the image of the cervix received by the colposcope into an attached CCD camera, using a beamsplitter that reflects part of the cervix image light into the CCD camera. The output of the CCD camera is digitized for each pixel of the CCD camera detector and input to a computer that performs various digital image processing operations on the image data to enhance the cervix image and/or make quantitative measurements of the features of the cervix.
In FIG. 5, numeral 10 designates a digital imaging colposcope having a dual eyepiece 11, an image magnification control knob 12, an objective lens 13, and an articulated support arm 18. Digital image processing is made possible by the addition of a CCD camera 19, the digital output of which is coupled to an image processing computer 60. Colposcope 10 contains an internal illumination source that produces an illumination beam 14 of white light that illuminates the cervix being visualized. The above mentioned portion of the colposcope 10 is known.
The most common way of detecting cervical or vaginal cancer is to perform a pap smear on the patient in the gynecologist's office. The pap smear sample is sent to a laboratory for evaluation, usually after the patient goes home. If a positive result is obtained from the evaluation, the patient must return to the gynecologist's office. The gynecologist then uses a prior art colposcope to locate the site at which a biopsy is needed. The gynecologist takes the biopsy tissue sample and sends it to a laboratory for analysis, usually after the patient leaves. If necessary, the patient is advised to return for whatever surgical, chemical exfoliation, or freezing treatment is recommended.
The major problems with the foregoing prior art techniques are the high cost and the amount of time required for three patient visits, and the fact that many patients become fearful and fail to comply with the gynecologist's instructions to return for more analysis, surgery, etc. Furthermore, the absence of accurate information on the size and area of cervical lesions may result in some gynecologists being influenced in favor of surgical treatment of HPV lesions because of the substantial fee for this service.
There is a very clear and strong need for a technique that will allow a gynecologist to examine the cervix, determine if any known lesions have increased in area enough to warrant surgical treatment, and to determine if the lesions consist of pre-cancerous or cancerous tissue, all in the same patient visit. A technique for measuring the area of a lesion of the cervix should be easily incorporated into common examination procedures and should be compatible with variations in equipment (such as colposcopes) used by various physicians. The technique should be non-invasive, and the accuracy of lesion area measurements should be sufficient to reduce errors in the measurement to approximately those of an orthogonally viewed lesion. Furthermore, the elapsed time between the physician's examination of the patient and the availability of lesion area measurement results should be minimal, perhaps a few minutes.
The possibility of erroneous determinations of the progression of a cervical lesion has been of great concern to physicians for a number of years. However, despite the great need for a practical solution to this problem, none has yet been proposed.
One characteristic of aceto-white lesions that has been found to be useful is the size of the lesion. (The application of dilute acetic acid or vinegar to the surface of the normally pink cervix will cause a transient white opacification of tissue that has been infected with human papilloma virus or if it is neoplastic. The white regions are particularly well visualized when observed with some magnification, such as occurs during a colposcopic examination, and are referred to as aceto-white lesions. They are significant since they aid in the diagnosis of pre-cancerous and cancerous lesions, allow for a direct visual estimate of the extent of the disease, and serve as visible guides to aid in the selection of the best areas of the cervix for biopsy.) The size of a lesion has been found to have a positive correlation with lesion severity. Large aceto-white lesions are more likely to be under-diagnosed than small ones and it has been suggested that size be used as a criteria for treatment. Despite this recommendation, the consistent and accurate determination of an aceto-white lesion size is not, as a practical reality, presently achievable.
It is also known that many aceto-white lesions may regress and disappear on their own. An objective means of measuring the area involved by a aceto-white lesion would provide a consistent means of evaluating a patient over a period of time. Early studies have indicated that such an approach might be feasible and beneficial, particularly in a pregnant patient.
However, it has been recognized that area measurements obtained using a computer graphical approach introduces an inherent error due to the computer image being a two dimensional representation of a three dimensional cervix.
As previously mentioned, digital colposcopy provides a convenient method to measure areas or distances in an image of the cervix. These measurements can be performed directly on a computer monitor by simply outlining or marking the area of interest using a computer input device such as a mouse or joystick. The area or distance can be automatically calculated by summing the number of pixels involved. Although this is a simple and fast procedure, it unfortunately may produce very misleading results since the image of the cervix on the monitor is a two-dimensional projection of a three-dimensional cervix. The measured distance or area can be either larger or smaller than the actual feature being measured, depending upon the angle of view. These errors in measurement can be quite large (up to nearly 500%) and are particularly worrisome if used for lesion surveillance. For example, during surveillance a change in viewing angle between examinations could result in an apparent shrinkage of a lesion when in reality the lesion has enlarged.
As previously explained, one method for correcting for errors due to use of a two-dimensional projection of a three-dimensional cervix is to determine the slant angle at each pixel of the image of the lesion to be measured using the shape-from-shading method. The inverse cosine of that slant angle can then be used to correct the area of each individual pixel. However, one limitation of this technique is that it assumes a particular lighting method that approximates a point light source and requires the use of a camera that has a linear response to the intensity of the light reflected from the cervix. Unfortunately, traditional film-based photography does not produce images that are linear in response to the reflected light and therefore are not suitable for this method.